TheBeastie wrote:
Yeah I recommend copper foil with one side being adhesive, like how it is here... and you place the copper over the points and then place the nibs on the inside (adhesive side) and push it down over the cell to insure good contact..

I had this idea inspired by your idea of using the self adhesive copper foil.
How about if we solder the copper tape before sticking it to the magnets. This way we avoid heat damage to the magnets and have good solid connection. Just a thought. My idea might be redundant, but i thought it would be worth sharing the idea.

TheBeastie wrote:
Yeah I recommend copper foil with one side being adhesive, like how it is here... and you place the copper over the points and then place the nibs on the inside (adhesive side) and push it down over the cell to insure good contact..

I had this idea inspired by your idea of using the self adhesive copper foil.
How about if we solder the copper tape before sticking it to the magnets. This way we avoid heat damage to the magnets and have good solid connection. Just a thought. My idea might be redundant, but i thought it would be worth sharing the idea.

I am assuming you want to add solder to increase thickness?
I used solder on the copper tape to connect the end terminal discharge wires but thats the only place, I had left enough spare distance copper on the ends to solder them on without affecting the magnets.

I first ordered "copper sheet" from ebay when first trying to build this pack but it was way too thick. So I gave up on that and just went to the copper foil that is single sided adhesive.
Probably one thing I wouldn't have minded doing with the build is had at least a tiny bit thicker foil.. I could of got away with something a tiny bit thicker..

To give you an idea of the thinness of the current copper foil I used, when I was using rough sandpaper on the bare 18650 terminals which left a lot of easily visible scratch marks and then placed the copper foil on top/nibed it and then pulled it off to examine the copper foil you could see the inverted etching of the marks from the 18650 terminals on the copper so it was clear there was serious contour and contact (good)..

I considered the idea of using chemicals to clean away one sheet of copper foil tape and then just double the layer so it was overall thicker but I didnt bother.

Maybe this copper foil below is the answer, its got no adhesive side so you can just add two layers of copper and glue the magnet into place on top..http://www.ebay.com/itm/1pcs-99-9-Pure- ... OxyrM5TIY1~
Titled "99.9% Pure Copper Cu Metal Sheet Foil 0.05 x 100 x 1000 mm #E3-0051" incase the ebay link dies..

I am still happy with my build. I checked the voltages of it at the end of my last complete ride/discharge last week and they were all very even..
Lately I been riding with PAS level 4 which draws about 500-600w of power and just purely dry pedaling in high gear so no effort at all from me except looking like I am pedaling and its still working fine.

I am still surprised by the level of some amps people try to squeeze out of 4P 18650 setups on soldered/spot welded packs that I think are beyond the rating of 18650s for long-term health of cells.. I think P4 is only good for 250w kits on 18650 cells for getting a lot of cycles in return.
If you want to build a pack like mine and beat the shit out of it with high amp draw you should do some testing before hand.

If you look at any decent 18650/20700 cell discharge/cycles chart you will see the heavier the discharge the faster the cell dies... Or another way to look at it is less total capacity due to abusive level of discharge as another graph for uneven voltages in cells/stuffed shitty battery pack..

Very high end 20700 cell here that's getting pretty shitty quite quickly due to excessive discharge even if its 'rated' for it.

If you look at a Tesla EV battery pack the connections between each 18650 cell are tiny. The reason Tesla can give a good warranty on the battery pack is because they aren't beating the hell out of the cells with heavy amp draw. Wanting overly thick connections on the cells can be viewed as bad engineering.https://chargedevs.com/features/a-close ... e-bonding/

Tushar D wrote:Hi Beastie
So would you suggest sanding the 18650 terminals or leave it as it is?

If we could stick the magnets to the copper sheet and place it on terminal we can easily remove it whenever we want. Would you suggest doing that?
Thanks

Don't know if you have read the whole thread but x-speed said it lowers resistance by about 10 times sanding and cleaning each cell terminal, I sanded and cleaned it with acetone.. Also if I wasn't going to put the copper on with in an hour or so of sanding and cleaning I would do it again before I resumed the pack build as the amount of micro corrosion gets unacceptable apparently, refer to x-speeds posts on it for more details.https://endless-sphere.com/forums/viewt ... 50#p935718

I used rough sandpaper mostly as I think it helps to get some mini groves in there but the down side is that it would provide spots for oxygen to sit in there if the copper/magnet doesn't squeeze them out when placing them on the bare cell terminals.

I can remove my "2S7P" single copper/magnets sets as a whole easily enough. the copper foil is strong enough to allow me to do that with no issues, the whole pack is wrapped in Kapton tape so I would have to remove that first..so far I have not needed to remove or change anything.

Last edited by TheBeastie on Sep 12 2016 1:24pm, edited 1 time in total.

TheBeastie wrote:
I am still surprised by the level of some amps people try to squeeze out of 4P 18650 setups on soldered/spot welded packs that I think are beyond the rating of 18650s for long-term health of cells.. I think P4 is only good for 250w kits on 18650 cells for getting a lot of cycles in return.

Actually, according to this research paper which is studying NCM cells, the sweet spot for longevity is actually 2C:

That means the ideal discharge rate for 4p 3Ah cells in a typical ~50V pack is 1200W .

On your copper foil, is it really .05mm thick? Wouldn't that create substantial heat? I may have done it incorrectly, but I calculated that even the .25mm copper that snath uses would create a few watts of heat in the pack, so I'm actually planning on using .5mm thick copper, which I should still be able to dimple and fold with pliers no problem. Perhaps this is entirely over-kill.

Im excited to do one of these DIY packs using sheeting and magnets. Are 3/8 x 1/8" Magnets and 12mils thick copper sheeting decided as the best materials at this point and not the nickel? I'm doing 12s4p and have pulled up to 50 amps peak and maybe 30 continuous. The 12mils copper is thick enough or what nickel do u recommend? Got lg chemicals 847 conductive paste but was thinking of plating copper with a very thin layer of silver to stop oxidationhttp://www.finishing.com/faqs/silverathome.shtml

I'm going to put it on the bottom of my skateboard so likely lots of vibration. I'd like to cover it in duct tape as well to hold everything in place. I'm still thinking it'll be a bit flexible one cell deep and just the copper, magnets, and duct tape and that's an appeal..I hope the flexibility won't be a problem and I'll test the pack resistace when I first set it up. Surprised more people don't do this. Assuming it'll work.

Ok last question! I sense math and a correlation between wire and sheeting ampacity. How does that work and is there consensus?

There's a thread on improving grease conductivity here but I haven't seen much posted.. I was expecting to see youtube videos with digital multimeters showing ohm resistivity etc but not much so far.https://endless-sphere.com/forums/viewt ... 14&t=61542
Looking back at such stuff while I was interested in it I didnt use any such compounds, was just too eager to have something built, but if I was to build another pack this stuff looks great.

That silver/nickel electroless plating solution looks very interesting but there seems to be almost nothing via google about the resistivity levels, would be interesting if you tried testing/measuring the ohms level of the stuff to see what it can do.

Last edited by TheBeastie on Sep 21 2016 3:48am, edited 3 times in total.

The contacting surface of positive terminal to copper strip is magnitudes larger the usual conducting strip cross section (at D of 7mm it is EDIT: 38mm2).
Strip plating should just be observed as corrosion protection for cooper, not contact conductivity enhancer. Electroplating a good layer of Nickel looks like better option to me. Nickel is stable, cheap and simple acid electrolyte (ex. vinegar) can be used for electroplating. Cyanides where used to kill people for centuries, I feel not comfortable with a cyanides jar in front of me.
Thin silver layer, you can achieve with past application, can result insufficient for real corrosion protection.

Last edited by parabellum on Sep 22 2016 9:25am, edited 1 time in total.

I was just looking on ebay for 'copper foil' and one of the annoying things that makes it hard to search is everyones selling the same thing its hard to just get a list of choices in general and often I think some sellers who sell so much stuff they get mixed up on similar items anyway..

Well I found "pickbestforyou" who sells everything including the kitchen sink has a wide variety of copper foil thicknesses if you search the user directly "Copper Sheet Foil". This brings up each individual option they have.http://stores.ebay.com/pickbestforyou/_ ... 1049175910

This was "Pure Copper Cu Metal Sheet Foil 0.2 x 200 x 1000", for me it was too thick there wasn't much contour to the cell terminal, maybe if you got tools to hammer/form the copper to perfectly contour to the cell terminals it could work for you.. I would prefer to buy thinner foil and multi-layer it until I get the desired thickness.https://www.youtube.com/watch?v=O2aoyt6arWg

I'll use .4mm copper. No one told me 50amps max is way too much for 12mil! It is right? Thanks for all your help. My max amps ever is 50 and mostly am much less

If the resistance is reduced by using the same thickness copper but using a wider strip.. How wide can you go till the resistance is no longer decreasing with wider strip?

I'm still waiting to see how the magnets, copper, and positive side will fit together but I don't want to have to indent the thick copper and think I should keep it flat. I have some paper rings to better seperate the terminals coming

Or how about one magnet per battery with the cells connected heat to tail? What's the most conductive magnet, has it been determined that a magnet is a good enough conductor!? maybe I can further plate one with a bunch more of something ? Tons of nickle.

TheBeastie wrote:
Yeah I recommend copper foil with one side being adhesive, like how it is here... and you place the copper over the points and then place the nibs on the inside (adhesive side) and push it down over the cell to insure good contact..

I had this idea inspired by your idea of using the self adhesive copper foil.
How about if we solder the copper tape before sticking it to the magnets. This way we avoid heat damage to the magnets and have good solid connection. Just a thought. My idea might be redundant, but i thought it would be worth sharing the idea.

I am assuming you want to add solder to increase thickness?
I used solder on the copper tape to connect the end terminal discharge wires but thats the only place, I had left enough spare distance copper on the ends to solder them on without affecting the magnets.

I first ordered "copper sheet" from ebay when first trying to build this pack but it was way too thick. So I gave up on that and just went to the copper foil that is single sided adhesive.
Probably one thing I wouldn't have minded doing with the build is had at least a tiny bit thicker foil.. I could of got away with something a tiny bit thicker..

To give you an idea of the thinness of the current copper foil I used, when I was using rough sandpaper on the bare 18650 terminals which left a lot of easily visible scratch marks and then placed the copper foil on top/nibed it and then pulled it off to examine the copper foil you could see the inverted etching of the marks from the 18650 terminals on the copper so it was clear there was serious contour and contact (good)..

I considered the idea of using chemicals to clean away one sheet of copper foil tape and then just double the layer so it was overall thicker but I didnt bother.

Maybe this copper foil below is the answer, its got no adhesive side so you can just add two layers of copper and glue the magnet into place on top..http://www.ebay.com/itm/1pcs-99-9-Pure- ... OxyrM5TIY1~
Titled "99.9% Pure Copper Cu Metal Sheet Foil 0.05 x 100 x 1000 mm #E3-0051" incase the ebay link dies..

I am still happy with my build. I checked the voltages of it at the end of my last complete ride/discharge last week and they were all very even..
Lately I been riding with PAS level 4 which draws about 500-600w of power and just purely dry pedaling in high gear so no effort at all from me except looking like I am pedaling and its still working fine.

I am still surprised by the level of some amps people try to squeeze out of 4P 18650 setups on soldered/spot welded packs that I think are beyond the rating of 18650s for long-term health of cells.. I think P4 is only good for 250w kits on 18650 cells for getting a lot of cycles in return.
If you want to build a pack like mine and beat the shit out of it with high amp draw you should do some testing before hand.

If you look at any decent 18650/20700 cell discharge/cycles chart you will see the heavier the discharge the faster the cell dies... Or another way to look at it is less total capacity due to abusive level of discharge as another graph for uneven voltages in cells/stuffed shitty battery pack..

Very high end 20700 cell here that's getting pretty shitty quite quickly due to excessive discharge even if its 'rated' for it.

If you look at a Tesla EV battery pack the connections between each 18650 cell are tiny. The reason Tesla can give a good warranty on the battery pack is because they aren't beating the hell out of the cells with heavy amp draw. Wanting overly thick connections on the cells can be viewed as bad engineering.https://chargedevs.com/features/a-close ... e-bonding/

Beastie I don't see how many amps you used. U use super thin copper. Doesn't it get hot at 600 watts? how many amps? I figured for every ten amps another .1 thickness

TheBeastie wrote:
I am still surprised by the level of some amps people try to squeeze out of 4P 18650 setups on soldered/spot welded packs that I think are beyond the rating of 18650s for long-term health of cells.. I think P4 is only good for 250w kits on 18650 cells for getting a lot of cycles in return.

Actually, according to this research paper which is studying NCM cells, the sweet spot for longevity is actually 2C:

That means the ideal discharge rate for 4p 3Ah cells in a typical ~50V pack is 1200W .

On your copper foil, is it really .05mm thick? Wouldn't that create substantial heat? I may have done it incorrectly, but I calculated that even the .25mm copper that snath uses would create a few watts of heat in the pack, so I'm actually planning on using .5mm thick copper, which I should still be able to dimple and fold with pliers no problem. Perhaps this is entirely over-kill.

Overkill with the .5mm?!?!? I was going to do .4 after looking at ampacity stuff but read .05mm being used at 600 watts.

Hummina Shadeeba wrote:
Overkill with the .5mm?!?!? I was going to do .4 after looking at ampacity stuff but read .05mm being used at 600 watts.

Your technically looking at it from a one? or is that two? dimensional point of view, from my point of view my copper is "6cm thick"... With sheet wiring you got to think a bit more about the surface area gain.. Even if the 18650 positive terminal has 8mm wide surface contact if I rolled up 8mm of sheet copper it turns into a decently thick wire.
Most copper wire is in stranded form to give more flexibility, cheaper stuff is single copper core. If you buy extra thin copper sheet foil you can get the same advantages in stranded wire in terms of flexibility as in you can multi-layer the copper sheet and get considerable more 'contour' to the 18650 terminal as well as have more thickness, theres just no point having thick single ply copper if you can't get good contact to the cell terminals.https://forums.tesla.com/en_AU/forum/fo ... anded-wire

While that conductive grease looks ideal to me, alternatively a more dodgy idea is to use a "silver conductive pen", there seem to be a fair amount of companies making this type of product probably the resistivity ohms/cm vary between the quality of each product. Also it only gets really conductive when it dries which is a bit of a problem if its tightly sandwiched between the copper and the cell terminal, maybe a tiny bit could help.

The lg chemicals stuff was sent so quickly I didn't have time to second guess and it's on route and I'll do a test to see how it does

But what about the magnet in the circuit? I think adding one to the positive side, then the copper, and then a second magnet would make it much easier and safer as I can then get the thick copper over the hump where it's so close to the rest of the can and the negative terminal. And if have good flat surfaces everywhere to make contact. Be neater than lots of thin layers (which I guess is what people are doing with the .05mm). Or could string a chain of batteries with a magnet between each? If only the magnet were conductive enough. I'll find out when they come I guess. 1/8x3/8" n50. But surely they could be made more conductive and that sounds doable if not already done

After I make the pack I'm planning to pot it all to some degree in some hard rubber! I'll just keep painting it with a 50 duro silicone and build up a quarter inch. It won't allow heat to transfer well but will hold everything in place and better handle vibrations and I can cut it if needed or stick my multimeter prongs through and wouldnt have to worry about oxidation if I use copper

am I right in thinking with 12s4p configured in a pppp s pppp s pppp s etc. string, that for the 4 parallel cells they will have very little current between all connections and it is just the ends of the group of 4, where they meet the next group, that's where the only higher series currents will occur?

How out of balance would be too far for putting all the cells in parallel to charge? But I'm set on using magnets for balance wires and covering it in rubber The rubber will hold it all together nicely I think. Not so versatile but waterproof and no corrosion of the copper I imagine.

how much pressure is needed with the greases I'm not sure. the magnets people are using here give about 7lbs of pressure continuously and reliably. I think you'll still need something to add a continuous reliable pressure with the grease. Maybe even lots of rubber bands would work.

I´ve been reading the forum for some time to find a setup that is suitable for used/disassembled laptop batteries, i have a bunch of them, thus needing a serviceable pack. Love this thread, big up for all you guys contributing!

Been reading the entire thread and thought it would be nice with a summary based on what i´ve picked up. Please fill me in and correct me if i´m wrong!

1. Battery holders: To get rigidity it is an advantage if the holders can take several or the entire amount of cells in the pack. Gluing holders will make it more rigid. Some people have tried hot glue instead of holders but ending up with holders in the end. A rigid pack will flex less and disturb the connections less. Personally i got interested in this setup: http://s.aliexpress.com/6vMVbQ3y

Less space and the cell ends are free for any set up of NIB´s and strips/sheets/foil. Note to self is that it might create higher risk of shortage due to the lack of any protection from the holders. Would be interested in what you think!

2. NIB size. Some where around 12mm dia, 2mm thickness of magnets (N52?) seem to do the job in most cases

3. Material and material thickness: Thinner material thickness more towards a foil thickness of 0.05mm seem to be flexible and soft enough to maintain good contact without adding resistance during use (vibrations etc). Preparations is light sanding of cell terminals and cleaning with some kind of solvent. Vaseline can be used to protect oxidation and maintain a low resistance connection. Other products has only been discussed but not tested if i remember correct. Copper foil is what have been used with success compared with nickel strips that are harder and have caused issues on the field. Although i think it was in an MTB environment the issue occurred. Please fill me in here! Not seen anyone tried "foil thin" nickel (maybe not available?) yet.

4. Really of importance to make sure the magnets will not be able to jump and short the pack. A protection/damage control strategy during build, under use and at rebuild should not be neglected. However no catastrophic failures reported.

Did this give us a rough summary of the engineering being made and shared in this tread?

as was already said and I'm experiencing the 3/8" maybe isnt as good as something smaller as it pulls to the side of the cell when on the positive.

the protective paper donuts I was going to add for more protection on the positive side space the copper too far away and wish there was another thinner material and I'm not going to use them.

I plan to just use one layer of the copper between the cell and magnet, so one wrap around the magnets and then folded layers on top. I'll do as many folded layers on top of the magnets as appropriate for 50 amps when I figure that out. I wonder how many layers between the magnet and cell people are doing.

I'm wising I'd gotten some magnetic shielding foil as well as I want to orient cells with magnets close to each other and they're interacting and I'd like to see how well that stuff would work. Maybe the interaction will be a benefit as I can orient magnets polarity to have apposing fields and add to the pressure on the cell but it adds a complication and jumping risk Id rather not have. I'll have to tape everything down and likely space things further apart.

be doing it tonight and I'll post how it goes. Hopefully someone can confirm one layer of the .05" under the magnet is good.

I'm sure there's potential for some cool designs. maybe a rubberized strip with the magnets and copper imbedded. was going to sink the whole battery in rubber but I'll have to tape everything down anyway so likely wont bother but if there wasn't the magnetic interference problem I have sinking everything in hard silicone would be an easy way to secure everything and still possibly have access with probes or a blunt knife to replace cells. wouldnt stick to the cell's wrapping or use polyurethane and it will stick.

I would prefer just using multiple layers of the "adhesive copper tape" stuff with the adhesive cleaned or something.. If you can build your pack with 18650s with no interference from cell brackets/holders you should be fine with 0.05 stuff, especially if your using extra contact greases, but using 0.05 sheet is at least for me considerably more difficult to get the positive side of the cells perfectly contoured/connected compared to using copper tape.

Last edited by TheBeastie on Nov 03 2016 3:53am, edited 1 time in total.

Are u doing just one strip of .05" for all the connections or is it folded over and how many times? How many amps do u do?

My batteries, magnets, and copper are still sitting here and waiting for the answer to that question..but much more so really waiting to finish the skateboard
With the cavity im still set on potting the battery in...w 50duro silicone.

I got some of the 'CHEMTRONICS CW7100 Silver Conductive Grease' and did a bit of testing with the DMM. I found an online seller in Melbourne who was selling it at a somewhat reasonable price.

While it wasn't lab level testing and done with a Chinese digital multimeter I am pleased with the results.
I placed a very small dab of it and let it be exposed to air over 24 hours and then tested, it still has a grease like nature to it after sitting exposed to air after 1 day.
I did test it straight after squeezing out a portion and the multimeter was all over the place when I stuck the probes into it..

From the small somewhat uneven dab I was able to get around 3.3ohms with the probes a few mm apart pushed down on the cardboard packaging.
I took some pics attached with my iPad in my other hand of the DMM and the probes touching the electrically conductive grease.
*Add/Edit* I tested another 24 hours later (so a few days in total) and was able to get around 1.9ohms so the dryer it gets the better it conducts.. Also note that holding my probes directly together shows 0.5ohms on my meter..

So logically if its around 1mm of gap or less of space between the copper covered magnet and the cell terminal then its definitely doing its job of keeping out air and filling the gap.. as having 1ohm or less resistance is most certainly better than infinite resistance of air. This stuff can only help as long as its in the right amount.. too much and your blocking metal on metal contact, too little and your allowing air to oxidize/ruin your existing metal contact and leaving space where the CW7100 could be allowing a bit more current to flow through.

CHEMTRONICS CW7100 Silver Conductive Grease

Add/Edit again..
So about a week later depending on where and how hard I press down on the probes I get between 0.8 and 1.4 ohms on the meter.. Also note again that holding the probes directly together gets 0.5ohms

0.8ohms.jpg (124.04 KiB) Viewed 1326 times

0.8ohms2.jpg (86.13 KiB) Viewed 2492 times

1.4ohms.jpg (119.66 KiB) Viewed 1326 times

Last edited by TheBeastie on Nov 01 2016 2:57am, edited 2 times in total.